It has been estimated that over sixty percent of the combat aircraft losses occurring since the 1960's can be attributed to use of infrared responsive surface to air and air to air missiles. Moreover the existence of newer more sophisticated generations of these missiles including the usually hostile SAM 16, SAM 17 and SAM 18 missiles is now known in the western world. Missiles of these latter types are understood to include countermeasures capabilities making the traditional hot flare and similar basic defensive measures against heat seeking missile attack of limited or little value. Although improved aircraft defensive measures based on laser energy sources have been used to some degree with respect to such missile weapons, until recently laser based infrared countermeasures have been laser source-limited, that is limited in both the available output power level and the spectral coverage achievable. In a very real sense therefore the missile and missile countermeasures battle scene has recently been biased in favor of the missile and its seeker by these power level and spectrum limitations.
As late as 1997 for example the best available solid state laser infrared source for missile countermeasures use operated in the range of five watts of average power level and provided little energy output in portions of the infrared spectrum known to be considered in the sensor of later design missiles. Although laser materials based on a certain class of chalcopyrite alloys have recently made it possible to exceed this 1997 power level by a factor of four and to achieve peak powers in the range of a hundred million watts per square centimeter in a nonlinear optical crystal material, even higher power levels and operation in yet inaccessible portions of the infrared spectrum are viewed as desirable improvements in the missile defense art. The present invention addresses this area of need and provides an infrared capability that is useful in areas other than the missile defense field.
Other needs for the present invention are also believed to exist within the military art. Following the decrease in tensions between major world powers in the 1990's, the threat of chemical or biological weapons used by smaller potential adversaries has emerged as a remaining and ongoing concern for the United States and other free world military forces. With regard to such chemical or biological weaponry it is known for example that one chemical warfare agent now available to most potential adversaries, i.e., the mustard gas of World War I infamy, provides a readily detectable and remotely sensible signature in a specific region of the infrared portion of the electromagnetic spectrum. This signature is, however, somewhat limited in bandwidth and therefore requires access to parts of the infrared spectrum which are not conveniently available with many laser sources. Similar limited spectrum signatures are believed to exist for other chemical and biological warfare agents. The remote, safe distance, sensing of such agents is of clear desirability in protecting the people and equipment necessary to a military operation. However the variety of threats posed by potential chemical and biological weapons now suggests that access to virtually unlimited areas of the infrared spectrum is desirable in the development of chemical and biological warfare defensive apparatus.
From a third perspective, an equal or perhaps even greater military interest in the infrared spectrum is prompted by the presence of windows of reduced atmospheric absorption located in certain specific bands of the infrared spectrum, especially for example in the 2-6 micrometer wavelength band and in the 8-12 micrometer wavelength band. These windows are believed to offer opportunity for communication, surveillance, and other military and civilian uses not currently considered feasible. The current situation in infrared spectrum applications may in fact be comparable with the somewhat recent advent of increased limited spectrum coverage and spectrum agility in the radar utilized microwave frequency parts of the electromagnetic spectrum, a development which has for example made spectral distinction between rain, snow and sleet possible in a weather radar system. In addition to military uses there of course exists numerous communication, detection and object-illumination applications in the non military world which can be benefited by efficient access to specific and possibly newly available portions of the infrared spectrum.
As a practical matter however infrared emitters usable in the most desirable infrared emission source, i.e., usable in the solid state stimulated emission coherent output devices; such as the semiconductor laser, generate outputs at certain specific wavelengths. These wavelengths are, moreover, separated by infrared and other spectral regions in which no desirable efficient direct emission source is available. The gas-based carbon dioxide laser is a non-solid state example of this situation in that such lasers are for example known to have strong emission lines residing at wavelengths of 9.3 and 10.6 microns. Emissions at wavelengths falling between these two wavelengths or at specific wavelengths above and below these wavelengths is significantly less.
The use of wavelength changing devices, devices based on the nonlinear optic characteristics of certain single crystal semiconductor materials, offers one approach for providing energy at otherwise inaccessible spectral locations. Prior to the early 1970's there was in fact little access to the wavelengths greater than 4 microns with the available ruby, Neodidium, YAG, Lithium, Argon and other laser materials of common usage--even with the use of the then available nonlinear and wavelength changing materials. In a similar manner, outside the infrared range an absence of sources in the 1 to 2.5 micron range of wavelength, especially for applications needing tunability, was difficult even when using wavelength mixing arrangements. The utility of a wavelength halving apparatus may be appreciated by, for example, considering that halving the wavelength (doubling the frequency) of the 10.6 micron emission line from a carbon dioxide laser provides an output at the wavelength of 5.3 microns, a wavelength at the extreme end of the two to six micrometer window where the most advanced missiles operate, a wavelength which is inaccessible to most laser materials.
The expression "nonlinear optic characteristics" when used in connection with the materials of such wavelength changing devices is generally understood to relate to the properties of crystal materials in which light transmission characteristics are intensity-dependent, i.e., materials in which the optical refractive index, n, is a function of the electric field strength vector, E, of the light wave. This representation is of course based on a Maxwell's equation model of light and the understanding that light energy is fairly described in terms of electric field strengths. The light wave index of refraction, n(E), is moreover represented as the sum of terms in an infinite series expansion of electric field strength vectors taken to the powers or exponents of zero, one, two and so on with each series term also including a factor of the form n.sub.0, n.sub.1, n.sub.2 and so on representing a refractive index. In mathematical symbols this relationship may be expressed as: EQU n(E)=n.sub.0 +n.sub.1 E+n.sub.2 E.sup.2 +n.sub.3 E+ (1)
or alternately as: EQU n(E)=n.sub.0 +.DELTA.n (1a) EQU .DELTA.n=2.pi./n.sub.o [.chi..sup.(2) E+.chi..sup.(3) E.sup.2 +.chi..sup.(4) E.sup.3 . . . ] (1b)
The material property of interest is .chi..sup.(2).
The zero exponent E term, i.e. the n.sub.0 term in the equation 1 series, corresponds to the refractive index used in traditional linear optics, the optics considered in entry level physics courses. The nonlinear materials of interest in the present invention are identified as chi two or second order nonlinear materials, an identification also based on this infinite series representation of the light wave n(E) and recognizing that these present invention materials are adequately characterized by a series of the equation 1 type which terminates with the third term, i.e. terminates with the second power of E, or E squared term.
The alloy Silver Gallium Selenide, AgGaSe.sub.2, in single crystal embodiment is presently considered the state-of-the-art carbon dioxide laser frequency doubling crystal, the preferred crystal for use in laser wavelength change devices such as an optical parametric oscillator, a second harmonic generator or a difference frequency generator (i.e., an OPO, a SHG or a DFG device; herein devices each referred-to simply as a "laser device"). For present purposes it may be considered that an optical parametric oscillator provides wavelength doubling or increasing action, the second harmonic generator provides a wavelength dividing or decreasing action and the difference frequency generator a sum and difference frequency mixture output. The term "laser device" is not herein limited to these specific wavelength changing arrangements however and may also identity other stimulated energy, coherent output apparatus. In other words the present invention is deemed not to be limited to a optical parametric oscillator, a second harmonic generator or a difference frequency generator.
As may be noted in the preceding and several other earlier paragraphs herein, the once universal convention of capitalizing the names of periodic table elements is observed in the present document. Additionally, the Silver Gallium Selenide, AgGaSe.sub.2, material is for example recognized as being formally classed as a "di-selenide" material. In the interests of brevity and simplicity however such formal reference is omitted herein and this material as well as the other similarly classifiable materials are herein referred-to by the shorter Silver Gallium Selenide and similar names.
The photon conversion efficiency of this AgGaSe.sub.2 state-of-the-art and most widely used infrared nonlinear optical crystal material is limited in wavelength doubling service because of its non optimal birefringence characteristic. In view of such birefringence limitation, laser apparatus use of this material results in a crystal phase matching angle failing to effectively utilize the available optical nonlinearity of the material, a phase matching angle also allowing excessive walk-off of the signal and pump beams within a AgGaSe.sub.2 crystal and the accompanying severe loss of photon conversion efficiency. The terms "birefringence" and "walk-off" are believed known in the art and are discussed and defined in some detail in the ensuing paragraphs of this disclosure. Relatively low thermal conductivity and the resulting thermal lensing tendency is another area of difficulty with this AgGaSe.sub.2 state-of-the-art nonlinear optic material. Yet another limitation of AgGaSe.sub.2 is excessive photon energy absorption at a wavelength of two microns, a limitation which limits its performance in two micron-pumped optical parametric oscillation-based laser systems.
Other nonlinear optical materials are of course available for possible use in overcoming these difficulties with Silver Gallium Selenide. Some such materials together with Silver Gallium Selenide are classified as chalcopyrite materials in a broad sense of the term chalcopyrite. Included in these other materials are for example Silver Gallium Sulfide, AgGaS.sub.2 ; Zinc Germanium Phosphide, ZnGeP.sub.2 and Cadmium Germanium Arsenide, CdGeAs.sub.2. With the possible exception of the first of these materials, known limitations of the material make these other materials even less desirable in practice for present need laser wavelength changing use and have therefore contributed to the AgGaSe.sub.2 material having its current state-of-the-art status. The Silver Gallium Sulfide, AgaS.sub.2, material, when modified into a somewhat related four element or quaternary alloy as disclosed herein, is deemed a viable and complementary, material for use in nonlinear optical apparatus, especially in view of the transparency in the red end of the visible wavelength portions of the optical spectrum it provides and the resulting wavelength-change coverage of an additional spectral region.
The Silver Gallium Selenide material is considered in significant detail in the first of the examples included in the present patent document. As related subsequently herein this detailed consideration of Silver Gallium Selenide is partly based on it being a nonlinear chalcopyrite material of close relationship to one of the quaternary alloys of principle focus in the present patent document--and therefore of interest in the present "closely related material" disclosure of this quaternary alloy. The consideration of Silver Gallium Selenide herein is also based on the fact that the properties of this three element or ternary alloy are in some specific characteristics similar to those of one focused upon quaternary material, ie., AgGa(Se.sub.(1-x) Te.sub.x).sub.2. Moreover the present document interest in the Silver Gallium Selenide ternary material is also based on the fact that it is a viable starting component for fabricating this one of the focused upon quaternary materials. Similar relationships are seized upon in the present patent document with respect to another focused upon quaternary material, Silver Gallium Sulfide, AgGa(Se.sub.(1-x) Te.sub.x).sub.2, as is described in detail in the following paragraphs and the examples disclosed below.
Returning to the present background of the invention discussion, in view of little more than the recited limitations of what is considered to be the state of the art best infrared wavelength changing material, there is clearly need in the laser apparatus art for a frequency doubling material offering a more desirable combination of performance characteristics than has heretofore been available. The present invention is believed to provide desirable answers for this need in the form of Tellurium-inclusive quaternary alloy chalcopyrite materials and. their utilizations. The present invention focuses on two Tellurium-inclusive alloys including the quaternary alloys Silver Gallium Selenide Telluride, AgGa(Se.sub.(1-x) Te.sub.x).sub.2 and Silver Gallium Sulfide Telluride (i.e., Silver Thiogallate Telluride), AgGa(Se.sub.(1-x) Te.sub.x).sub.2. These quaternary alloys, are considered relevant over a range of Selenium/Tellurium and Sulfur/Tellurium compositions as is indicated by the complementary x subscript notations in these chemical formulas. The present invention is however deemed not to be limited to these specific Tellurium alloys.